Heating furnace



W. L. SCHMIELER HEATING FURNACE Sept. 27, 1966 Filed June 25. 1965 2 Sheets-Sheet 1 Sept. 27, 1966 w. L. SCHMIELER HEATING FURNACE 2 Sheets-Sheet 2 Filed June 23. 1965 mvsmon.

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5 i /1 TTOANE) United States Patent 3,274,991 HEATING FURNACE William L. Schmieler, 19609 Winslow Road,

Shaker Heights, Ohio 44122 Filed June 23, 1965, Ser. No. 466,183 r 19 Claims. (Cl. 126110) This application is a continuation-in-part of my copending application Serial No. 309,607, filed September 18, 1963, and entitled Heating Furnace, now abandoned.

This invention relates to heating furnaces and particularly to heating furnaces of the type having a heat exchanger in which the products of combustion are recirculated during the operation of the furnace.

For purposes of illustration, the invention is described as applied to a household heating furnace, its use in connection with other types of furnaces being apparent from the illustrative example.

Household heating furnaces generally are subject to large stack losses because they employ natural stack drafts wherein the flue inlet is connected with the top portion of the heat exchanger. Consequently, even though heat is not required in the space to be heated, the induced draft causes a continuous flow of outside air into the heat exchanger and out through the stack, and therefore wastes the heat stored in the exchanger.

In the present furnace, provision is made for the recirculation of the products of combustion within the heat exchanger, and the bleeding off of a fractional part only of the products from a location near the lower level of the heat exchanger. This reduces the stand-by heat losses from the exchanger to a negligible amount.

Furthermore, effective introduction of burning gases into the exchanger and an effective recirculation of the products of combustion within the exchanger in sweeping relation to the interior faces of the exchanger walls are obtained so that the heat is more uniformly distributed, hot spots and the like are eliminated, and higher efficiency of heat exchange between the exchanger and outside air to be heated results.

The burning gases are introduced in such a manner that they do not impinge directly on the wall of the container opposite the discharge end of the burner, but instead are directed radially outwardly into a surrounding annular stream of products of combustion being recirculated through the exchanger.

The forces of the recirculated products of combustion are employed for inducing an adequate inflow of secondary air for combustion and for reducing back pressure on the hydrocarbon fuel burner used for firing the furnace.

A recirculating blower is provided of which the rotor is disposed directly in the recirculating gases and becomes heated by the high temperature thereby, but the rotor shaft by which the rotor is driven by an external motor or drive shaft is so shielded that no objectionable amount of heat is transferred therethrough to the driving mechanism, bearings and the like. 7

The invention includes a cabinet type furnace wherein, due to the shape and arrangement of the cabinet walls and those of the heat exchanger, the exterior of the heat exchanger is more effectivelywashed by the air stream for heating space to be heated, and undesirable heating of the exterior of the cabinet is reduced to "a minimum.

Various objects and advantages will become apparent from the following description, whereinreference is made to drawings which illustrate apreferred embodiment of the invention and wherein:

FIG. 1 is a front elevation of a furnace embodying the principles of the present invention, the removable cover 3,274,991 Patented Sept. 27, 1966 for the front of the outer cabinet being omitted for clearness and illustration;

FIG. 2 is a top plan view of the structure illustrated ice in FIG. 1;

FIG. 3 is a vertical sectional view taken on the line 33 in FIG. 1;

FIG. 4 is a vertical fragmentary sectional view through heat exchanger and associated burner parts of the furnace illustrated in FIG. 3, and is taken on line 4-4 in FIG. 3; and

FIG. 5 is an enlarged fragmentarycross sectional view of the burner, and is taken on line 55 in FIG. 3.

Referring to the drawings, the furnace, indicated generally at 1, comprises essentially a heat exchanger 2 into which products of combustion are introduced and through which they are continuously recirculated.

For the purpose of illustration, the furnace is described herein with the heat exchanger in its preferred upright position, its use in other positions being apparent from the illustrative example.

The exchanger 2 is preferably made of sheet metal. It comprises an upright front wall 3 and an upright rear wall 4, these walls being in spaced face to face relation to each other, and having disposed therebctween, in face to face spaced relation to each, an upright partition wall 5. A, peripheral wall 6 of the exchanger extends about the peripheral edges of the front wall 3 and rear wall 4 and the outer peripheral lateral and bottom edges of the intermediate wall 5.

The front and rear walls are preferably ovaloid with the larger diameter end at the top so that the bottom portion of peripheral wall 6 is concave upwardly and is smaller in, diameter than the upper portion of the wall 6 which is concave downwardly.

The partition wall 5 is parallel to the front and rear walls and divides the interior of the exchanger into a forward portion between the front wall 3 and partition wall 5 and a rearward portion toward the rear wall 4 and the partition wall 5. The partition wall 5 has an inlet opening 7 near the top and an outlet opening 8 near the bottom. The space between the front wall 3 and the partition wall 5 connects with the openings 7 and 8 and provides a recirculating duct indicated generally at 9.

In order to supply burning hydrocarbon fuel into the heat exchanger, a combustion chamber 12 is provided. The chamber 12 is preferably in the form of a cylindrical shell, and extends across the space between the front wall 3 and partition wall 5, and preferably protrudes a short distance into the space between the partition wall 5 and the rear wall 4. The chamber 12 has an open outlet or discharge end 12a. The outlet end 12a is arranged in coaxial relation to the inlet opening 7 in position to discharge therethrough toward the rear wall 4 thereof, and is of less diameter than the inlet opening 7 so as to permit passage of products of combustion from the recirculating duct 9 around the exterior combustion. chamber 12 and thence, as an annular stream, through the opening 7.

In the form illustrated, a burner 13 is provided and is shown as one for gas fired furnaces, the use of an oil burner being readily apparent from the illustrative example. The burner 16 comprises a simple open ended tube 14 into one end of which fuel is fed in coaxial relation therewith and endwise thereof by a suitable gas nozzle 15. The nozzle -15 is arranged in spaced relation to the periphery of the open inlet end of the tube and is connected to a suitable gas supply line, not shown. A disc spreader 16 is provided at the open discharge end of the tube 14 for spreading the flame and products issuing from the tube. This discspreader is supported by suitable means, suchas a rod 17a carried on a suitable spider {17b which spider is secured in the discharge end of the tube 14. The disc spreader is of larger diameter than the discharge end of the tube,-is concavo convex, and is arranged with its convex face facing and spaced from the discharge end of the tube 14.: The spacing of the spreader '16 is such that it allows relatively free escape of gas and primary air from the discharge end of the tube 14 while deflecting the gas and air generally radially of the tube for impingement on the surrounding annular stream of recirculating products issuing from the inlet opening 7 of the wall 5.

In order to improve the mixture of primaryair and gas,

2 an annulus "18 which is of three dimensional reticulated body of metal fibre or strands arranged in net formis dis-' posed in the dischargeend of the tube 14 with its outer peripheryjuxtaposedVagainst the inner peripheral wall of the discharge end of the tube and its inner periphery exposed in coaxial spaced relation to the axis of'the tube close to the inner periphery of the tube. Such an annulus, with a relatively large central opening and a large number of crisscrossing fibres which provide an heterogenous arrangement of passages therethrough, tends to break down the laminar flow of gases and air and to cause them to mix more elfectively with those issuing through the annular space between the discharge end of the chamber 12 and the periphery of the inlet opening 7. By thus permitting enough seepage of primary air and gases through the body of the annulus 18, the usual orifice effect or vena contracta, such as would be obtained by a mere unperforate annulus which reduced diameter discharge opening, is avoided.

Primary air for the gas is readily induced between the nozzle 15 and the periphery of the rear end of the tube "14. The conventional electrode -19, which may be connected to a suitable power source, is provided for lighting the burner.

The burner is mounted to protrude into the chamber 12 in coaxial relation to an opening 20 in the front wall 3 of the combustion chamber 12. The opening 20 may be annular and larger in diameter than the tube 14 so that the secondary air can flow into theouter end of the chamber 12 between the periphery of the opening 20 and the exterior of the tube'a14. The burner 13 is mounted in a housing 21 and has an outer end wall 22 with air inlet ports 23 therein through'which both primary and secondary air are admitted into the interior of the housing 21. Accordingly, the air thus supplied passes, through the tube 14 as primary air and through the opening 20 in surrounding relation to the tube 14 as secondary air.

As mentioned above, the combustion chamber 12 is of smaller diameter than the opening 7 so that an annular passage, indicated at 24, is provided between the outer periphery of the combustion chamber 12 and the peripheral edge of the opening 7 in the wall 5.

A sleeve or duct- 25 is, connected at one end of the outlet opening 8 of the wall and extends into the recirculation duct 9 and is connected at its opposite end with the inlet of a centrifugal blower 26 which is operative to draw products of combustion from the space between the walls 4 and 5. The blower 26 is a conventional .typeof centrifugal blower having the usual scroll or casing 27 and a rotor 28 having a shaft 29. The blower and the scroll are arranged in the recirculating duct 9.

The duct 25 connects with the inlet of the'blower so that the blower receives the air axially of the rotor thereof.v

The air discharged from thescroll is discharged generally tangentially of the rotor, 28. For this purpose the scroll 27. extends laterally of the recirculating duct 9 and through a The opening 31 is arranged to discharge upwardly into the recirculating duct means between the Walls. 3 and '5. The opening 31 preferably is as wide as the scroll in a direction endwise of the rotor axis, and is large enough to permit the major portion of the products discharged from the scroll to pass into the recirculating duct 9. The shaft 29 which extends through an opening 3a in the front Wall 3 and at a location forwardly of the front wall 6 is mounted on a coaxial shaft 32 of a suitble electric motor '33.

Thus upon operation of the blower,.the products of combustion are forced upwardly through the recirculating duct 9, formed by the walls 3 and 5, and then impinge on the 1 bottom of the combustion chamber 12, pass clockwise and counterclockwise; thereabout in surrounding relation thereto and also forwardly through annular opening 24., Thus they form an annular stream of products flowing toward the rear wall 4'about the outletof the combustion chamber1 2. As mentioned, due to the arrangement of the spreader 16 and its spacing from the outlet end of the tube 14, the primary air and burning gases from the tube .14 spread outwardly, generally radially into the annular stream and thereby arepart-ially mixed therewith and carried along therewith, being to a large extent surrounded by a stream or layer ofrecirculating products issuing.

through the annular opening. Due to the downward pull of the blower, the products entering through the opening 24 immediatelystart to pass downwardly so that a portion of them passing toward and downwardly along the rear wall 4 of the exchanger form a layer. against which the products currently being discharged impinge and become mixed, and thereforedo not impinge directly on the rear wall 4. Consequently, hot spots and deleterious heating effects on the portion of the wall 4 in alignment with the outlet of the combustion chamber 12 are eliminatedprevent direct impingement of the currently discharged products from the tube 14, as deflected by the deflector 16, from direct impingement on the peripheral wall 6.

The recirculating products passing through the annular opening 24 produce a sub-atmospheric pressure and a Venturi effect at the outlet of the chamber 12 which assists in .the discharge of the products from the burner 13 and The structure thus described is enclosed in a cabinet 35.

The cabinetjZvS has a horizontal lower partition wall 36 beneath whichis mounted an air blower 3 7 driven by a motor 38. The cabinet has openings 39 through which air to be heated is drawn into the air blower 67. The airis discharged by the air blower 37 upwardly through an opening 40 in the wall 36. Y

The walls of the cabinet 35 are spaced from the heat exchanger. i 7

As illustrated in FIG. 1, this discharged air sweeps the exterior peripheral surface of the top, bottom, front, rear, and side wallsof the exchanger-2.

Due to the shape of the heatexchanger 2, the'width of the passage for air between the walls of the cabinet 35 and the side wallsr6 of the heat exchanger 2 decreases-upwardly. This causes the air to be heated to pass in inti-, mate contact with the walls of the exchanger.

Thepcabinet has an air discharge opening41 at thetop through which air is discharged into the space to be heated or intosuitable distribution ducts leading therefrom;

the wall 42 and enclosing the burner housing 22 and motort3 3.

,As mentioned, it is desirable that the heat exchanger2 be substantially air locked in standby condition, and for this purpose the bleeder duct 30 is connected to the interior of the recirculating duct 9 near the bottomof the duct 9 and leads into an auxiliary heat exchanger 45, as

Also, the surrounding recirculating products heretofore mentioned. The auxiliary heat exchanger 45 is coextensive in height, and in depth in the direction forwardly and rearwardly of the heat exchanger 2, with the heat exchanger 2. It is disposed in upright position between one side wall 6 of the heat exchanger 2 and the adjacent side wall of the cabinet so that air from the blower 37 can sweep the exterior of the auxiliary exchanger and recuperate heat therefrom. This exchanger 45 is connected by suitable pipe 46 to an outlet duct 47. The duct 47 preferably is connected to a stack which exerts only a slight stack draft.

With this structure the auxiliary heat exchanger 45 in addition to providing its function as an auxiliary exchanger combines with the duct 30 and pipe 46 and outlet duct 47 to provide bleeder duct means which lead from the exchanger to the atmosphere and which is operative to bleed to the atmosphere a fraction of the products being recirculated in the exchanger.

If desired, a suitable damper 48 can be provided in the bleeder duct 30 for regulating and balancing the amount of the products to be bled out of the recirculating duct 9 so as to maintain a proper balance with the fresh products being delivered into the heat exchanger 2 through the combustion chamber 12.

As mentioned one serious problem presented is preventing the transfer of heat from the blower 28 through its shaft 29 to the support for the shaft, which, in the form illustrated, is the motor 33. In order to prevent undue transfer of heat,'a sleeve 50 is provided and extends from the front wall 3 of the exchanger to the wall 42 of the cabinet 35. The sleeve 50 is connected to the walls 3 and 42 at its ends, respectively, and is in surrounding radially spaced relation to the shaft 29. Since it bridges across the space between the wall 42 of the cabinet and the front wall 3, it is swept externally by air from the air blower 37. The sleeve 50 is preferably frusto-conical with its small 'base connected to the wall 3 and its large base connected to the wall 42, so that the minimum entry area for conduction of heat into the sleeve from the front wall 3 is assured. Since the larger base is connected to the wall 42, a large path of heat conduction from the sleeve 50 into the wall 42 is provided. Also, the frusto-conical shape assures a larger peripheral surface area which is swept by the air from the blower 37.

The opening in the wall 3 through which the shaft 29 extends is indicated at 3a. This opening is preferably of only slightly larger diameter than the shaft 29 so that no appreciable amount of air can pass through the clearance space between the edge of the passage 3a and the shaft 29 into the recirculation duct 9. Such infiltration of air would cause further localized combustion and extensive heat at the blower, tend to unbalance the system and adversely affect the operation of the furnace. -If desired, a small asbestos seal 53 may be employed to assure that no infiltration of air occurs around the shaft. The opening 42a in the wall 42 may be coextensive with the larger base of the sleeve .50 so that air can circulate about the portion of the shaft 29 within the sleeve as well as about the portion of the shaft 29 which is forwardly of the wall 42, thus assisting in preventing heat from reaching the support motor shaft 32 and the motor bearings thereof.

With the arrangement illustrated it has been found that the amount of heat conducted through the shaft 29, even though the blower is at relatively high temperature, is so small that the temperature of the supporting motor shaft 32 and its bearings remain well within the normal operating temperature range.

It is desirable that the furnace be adapted for different recirculation capacities. However, it is rather expensive to change the structure for each different capacity, accordingly the blower is such that blades may be removed; for example, from every other blade to every fifth blade, thereby maintaining the proper mechanical balance and recirculating capacity without having to change the rotor 28, its mountings, and the motor 33. While the rotor is not as efficient with some blades removed, it is, in this condition, nevertheless efiicient enough so as not to contribute appreciably to the cost of operation.

It is apparent, therefore, that with the present structure, very effective flow of products of combustion along the inner walls of the heat exchanger and of the air about the exterior walls of the heat exchanger are provided, the interior of the walls of both exchangers being subjected effectively to the heated products without the creation of hot spots, and the exterior of the walls being effectively air washed.

The partial air locking of the interior of the heat exchanger so that the heated products cannot be drawn out readily, assures a large reduction in possible heat loss during standby conditions, and hence higher efficiency in the supply of heat to the space to be heated.

The terms upright and horizontal where used herein and in the claims are used in a relative sense for briefness in defining relative positions and movements of the various parts and not as absolute, as many of the advantages of the structure can be obtained when it is placed in other than an upright position.

Having thus described my invention, I claim:

' 1. A furnace comprising a hollow heat exchanger having a peripheral wall, and front and rear walls, respectively, and an intermediate partition wall between, and in spaced face to face relation to both the front and rear walls, said intermediate wall having an inlet for products of combustion near one end and an outlet for said products near the other end, the space between the front wall and intermediate wall providing a recirculating duct connecting the outlet and inlet;

a combustion chamber disposed in said space at said one end and having an open discharge end which is aligned with said inlet and directed toward the rear wall;

said chamber having a peripheral enclosing wall spaced from the peripheral wall of the exchanger so that products from said duct can impinge on one side of said peripheral wall of the chamber and flow in surrounding relation thereto, said peripheral wall of the chamber at the discharge end being arranged in spaced relation, transversely of the discharge end, to the periphery of the inlet so as to form therewith an annular passage for discharging toward the rear wall;

burner means for discharging hydrocarbon fuel into the chamber in a direction toward the open discharge end, means having air induction ports connected directly to the atmosphere and arranged to permit induction of air into the chamber for sustaining cornbustion therein;

a power driven recirculating blower connected with the interior of the exchanger so as to recirculate products of combustion from said outlet into surrounding relation to the chamber and thence through said inlet;

bleeder duct means in communication with the exchanger and operable for bleeding to the atmosphere a fraction of the products being recirculated; and

the discharge end of the chamber and periphery of the inlet of the heat exchanger being spaced apart and arranged so that the recirculated products blown through the duct, combined with the bleeding of products of combustion into the atmosphere, create a low pressure zone around the discharge end of the chamber.

2. A furnace according to claim 1 wherein said front wall, rear Wall, and intermediate partition wall are upright, the peripheral wall extends from the front wall to the rear wall, said inlet is near the top of the exchanger, said outlet is near the bottom of the exchanger, the recirculating duct is upright, the peripheral enclosing wall of the chamber is spaced from the top, bottom, and side walls of the exchanger so that the impingement of the products is on the lower portion of the peripheral wall of the chamber and the flow of products in surrounding rela-, tion is upwardly, and the inlet of the bleeder duct means is at a level near the level of the said outlet.

3. A furnace according to claim 1 wherein an air receiving enclosure is provided, the heat exchanger is disposed therein in spaced relation to the walls of the enclosure;

power operated air blower means are provided and are arranged to blow air through the enclosure in sweeping relation to the exterior of the exchanger;

one of said enclosure walls is in front of, and in spaced relation to, said front wall of the exchanger and provides an air space for circulation of air from the air blower;

, said front wall and said one enclosure wall have'aligned openings therethrough;

the recirculating blower is disposed in said duct and in- V cludes a rotor and a rotatable supporting shaft;

said shaft extends through said openings whereby an annular passage is provided between the shaftand the opening in said front wall;

a motor is disposed in front of said one enclosure wall and is drivingly connected to said shaft; and

means are provided to close said annular passage between the shaft and periphery of the opening in the front wall.

4. A furnace according to claim 3 wherein said last mentioned means comprises an irnperforate sleeve in surrounding spaced relation to the shaft and connected at its inner end to said front wall and at its outer end to said one wall, and bridging across said air space therebetween, and said sleeve is open to receive air at its outer end.

5. A furnace according to claim 4 wherein the sleeve is frusto-conical and has its smaller end disposed at the front wall and its larger end disposed at the said one wall.

6. A furnace according to claim 1 wherein the burner includes spreader means to cause the burning products initially discharged from the chamber to spread laterally of the direction of discharge and impinge on the annular stream of recirculated products before the products impinge on the rear wall.

7. A furnace according to claim 1 wherein the chamber terminates at its discharge end close to the plane of said intermediate Wall.

8. A furnace according to claim 1 wherein the discharge end portion of the chamber has a cross section smaller than the inlet in the intermediate wall and extends through said inlet and forms with the periphery of said inlet an annular passage extending radially of said inlet in surrounding relation to the peripheral wall of the chamber.

9. A furnace according to claim 8 wherein the chamber terminates at its discharge end rearwardly from, and close to, the intermediate wall.

10. A furnace according to claim 1 wherein said front, rear, and intermediate walls are upright, the chamber is disposed with its axis horizontal, the blower is disposed with its axis horizontal, said inlet in the intermediate wall is near the top and the outlet therein is near the bottom, and the bleeder duct means has its inlet in communication with the exchanger at a level near said outlet.

11; The furnace according to claim 1 wherein air blower. means are provided for sweeping the exterior of the exchanger with air to be heated, an auxiliary hollow heat exchanger is provided and is disposed alongside the first mentioned exchanger and in position to be swept by part of the air from said air blower means, and said aux-' iliary heat exchanger is connected at one end to the lower portion of the first mentioned exchanger so as to receive bled-off products therefrom, and is connected at its other end to the bleeder duct means for discharging there through the bled-01f products.

12. The furnace according to claim 11 wherein the recirculating blower includes a blower rotor, a scroll .enclosing the rotor and disposed in the duct, said scroll having an end wall with an inlet coaxial with the' rotor and connected to the outlet in the intermediate wall, and having a peripheral wall with outlet means therein, and means are provided at the outlet means to divide the stream of products discharged from the-recirculating blower and to direct partof the recirculated products from the blower the other end, is of gradually decreasing crosssection in-teriorly and .exteriorly.

14. A furnace according to claim 1 wherein the intermediate wall and rear wall are substantially parallel, and the width of the exchanger from one peripheral wall portion to the opposite peripheral wall portion is gradually decrescent from said one end toward the other end.

15. A furnace according to claim 1 wherein the walls of the exchanger are so arranged that its interior isovaloid in the longitudinal section, in a plane parallel-to the rear wall. a

16. A furnace according to claim-15 wherein the interior is substantially unobstructed fromone end to the other.

17. A furnace according to. claim 1 wherein the burner means comprises a tube havinga front end and rear end, said tube being open at both ends, a fuel nozzle at therear end having a discharge orifice arranged in coaxial relation to the tube and positioned to discharge into the tube axially thereof toward the front end, means for admitting primary air into the tube, a spreader disc at the front end of the tube, said disc being concave-convex with its convex face facing toward said front end, and means-supporting the disc fixedly in coaxial relationwith the tube and spaced forwardly fromsaid front end a distance to provide a relatively free passage. for escape .of fuel and air from the tube between the disc and front end of the tube and for deflecting said fuel and gas generally radially of the tube through said free passage.

18. A furnace according to claim 17 wherein the disc is of greater diameter than the tube. v

19. A furnace according to claim 17 wherein an annulus is provided and, is disposed in, and coaxial with, the tube adjacent the front end, said annulus being of three dimensional reticulated net material having its outer periphery disposed at the inner wall of the tube and its inner periphery coaxial with the tube and spaced radially inwardly from said inner wall.

References Cited by the Examiner UNITED STATES PATENTS 1,943,053, 1 1/1934 Boisset 126l16 3,152,633 10/1964 Morrison. JAMES w. WESTHAVER, Primary Examiner. 

1. A FURNACE COMPRISING A HOLLOW HEAT EXCHANGER HAVING A PERIPHERAL WALL, AND FRONT AND REAR WALLS, RESPECTIVELY, AND AN INTERMEDIATE PARTITION WALL BETWEEN, AND IN SPACED FACE TO FACE RELATION TO BOTH THE FRONT AND REAR WALLS, SAID INTERMEDIATE WALL HAVING AN INLET FOR PRODUCTS OF COMBUSTION NEAR ONE END AND AN OUTLET FOR SAID PRODUCTS NEAR THE OTHER END, THE SPACE BETWEEN THE FRONT WALL AND INTERMEDIATE WALL PROVIDING A RECIRCULATING DUCT CONNECTING THE OUTLET AND INLET; A COMBUSTION CHAMBER DISPOSED IN SAID SPACE AT SAID ONE END AND HAVING AN OPEN DISCHARGE END WHICH IS ALIGNED WITH SAID INLET AND DIRECTED TOWARD THE REAR WALL; SAID CHAMBER HAVING A PERIPHERAL ENCLOSING WALL SPACED FROM THE PERIPHERAL WALL OF THE EXCHANGER SO THAT PRODUCTS FROM SAID DUCT CAN IMPINGE ON ONE SIDE OF SAID PERIPHERAL WALL OF THE CHAMBER AND FLOW IN SURROUNDING RELATION THERETO, SAID PERIPHERAL WALL OF THE CHAMBER AT THE DISCHARGE END BEING ARRANGED IN SPACED RELATION, TRANSVERSELY OF THE DISCHARGE END, TO THE PERIPHERY OF THE INLET SO AS TO FORM THEREWITH AN ANNULAR PASSAGE FOR DISCHARGING TOWARD THE REAR WALL; BURNER MEANS FOR DISCHARGING HYDROCARBON FUEL INTO THE CHAMBER IN A DIRECTION TOWARD THE OPEN DISCHARGE END, MEANS HAVING AIR INDUCTION PORTS CONNECTED DIRECLTY TO THE ATMOSPHERE AND ARRANGED TO PERMIT INDUCTION OF AIR INTO THE CHAMBER FOR SUSTAINING COMBUSTION THEREIN; A POWER DRIVEN RECIRCULATING BLOWER CONNECTED WITH THE INTERIOR OF THE EXCHANGER SO AS TO RECIRCULATE PRODUCTS OF COMBUSTION FROM SAID OUTLET INTO SURROUNDING RELATION TO THE CHAMBER AND THENCE THROUGH SAID INLET; BLEEDER DUCT MEANS IN COMMUNICATION WITH THE EXCHANGER AND OPERABLE FOR BLEEDING TO THE ATMOSPHERE A FRACTION OF THE PRODUCTS BEING RECIRCULATED; AND THE DISCHARGE END OF THE CHAMBER AND PERIPHERY OF THE INLET OF THE HEAT EXCHANGER BEING SPACED APART AND ARRANGED SO THAT THE RECIRCULATED PRODUCTS BLOWN THROUGH THE DUCT, COMBINED WITH THE BLEEDING OF PRODUCTS OF COMBUSTION INTO THE ATMOSPHERE, CREATE A LOW PRESSURE ZONE AROUND THE DISCHARGE END OF THE CHAMBER. 